Toilets revolting; and that's an understatement

• "Not too bad" for "very good"
• Figure of speech like "no mean feat"
• Literary understatement
• Ironical understatement
• Understatement for effect
• 'Not unhappy,' e.g

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
rhetorical figure in which an affirmative is expressed by the negative of its opposite, from Greek litotes , literally "plainness, simplicity," from litos "smooth, plain, small, meager," from PIE root *(s)lei- "slimy, sticky, slippery" (hence "smooth"); ...

Wiktionary Word definitions in Wiktionary
n. (context rhetoric English) A figure of speech in which the speaker emphasizes the magnitude of a statement by denying its opposite; a figure of speech in which understatement is used with negation to express a positive attribute; a form of irony

Wikipedia Word definitions in Wikipedia
In rhetoric , litotes (, or ) is a figure of speech wherein understatement is used to emphasize a point by stating a negative to further affirm a positive, often incorporating double negatives for effect. For example, "She's not bad looking" could be used ...